Question

public class Guess {
    public static void main(String[] args){
        <sometype> x = <somevalue>;
        System.out.println(x == x);
    }
}

i have to change sometype and somevalue so that it returns false? is it possible?

Answer 1

One:

float x = Float.NaN; 

Two:

double x = 0.0/0.0;

Why?

As mentioned here already, NaN is never equal to another NaN – see http://java.sun.com/docs/books/jls/second_edition/html/typesValues.doc.html

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So why this is not returning false?

Float x = Float.NaN; 

The answer is that here, instead of a primitive assignment, there is a reference assignment. And there is a little auto boxing in the background. This is equal to:

Float x = new Float(Float.NaN); 

Which is equal to:

Float x = new Float(0.0f / 0.0f); 

Here x is a reference to a Float object, and the == operator tests reference equality, not value.

To see this returning false as well, the test should have been:

x.doubleValue()==x.doubleValue();

Which indeed returns false